The site is described as "Census microdata for social and economic research". It's an excellent and little known web site that is dedicated to collecting and distributing US census data. Its goals are to:

But as the column implies, the census taker was instructed not to write in column 26D so in this case the instructions can't help much.Column 26D was filled in by the staff at the census bureau after the census taker completed the task.The codes written in the column were to assist them in tabulating statistical information which they reported to Congress.

Unfortunately these numbers really don't provide additional information because they just reflect what was written in the Occupation columns by the census taker

GEM: Agricultural Census (Non-Population Census)Farm # - It refers to the Special census called the Farm ScheduleListen to Family Tree Magazine podcast #12 Census secrets with Curt Witcher for more information.